The increase in the concentration of carbon dioxide in the atmosphere is largely responsible for global warming. CO2 of human origin is essentially emitted into the atmosphere through the burning of fossil fuels in power stations and in a certain number of industrial units such as cement works, hydrogen production units or even steelworks.
In the context of reducing the emissions of greenhouse gases and/or the production of CO2 used for enhanced oil recovery, a unit that captures and purifies CO2 by cryogenic means can be used downstream of the CO2-emitting plants. The cryogenic capture and purification of CO2 are essentially based on the partial condensation of the CO2 at temperatures close to its triple point, which may be supplemented by one or more distillations to increase the CO2 purity of the end product. In order to perform these partial condensations and distillation, it is going to be necessary to compress the gases that are to be purified, to dry them and then to cool them in order to form a CO2-enriched liquid phase and a gaseous phase enriched in incondensable gases that will be separated in one or more partial condensation pots. With this kind of method, capture efficiencies of between 80 and 95% are achievable.
The liquid CO2 obtained is usually separated into at least two streams. The first stream is vaporized directly against the gas that is to be purified to supply the frigories at the highest temperatures. The second stream is expanded before being vaporized so as to supply the frigories necessary for cooling the gas that is to be purified to the lowest temperatures of the cryogenic part, namely to temperatures close to the triple point of CO2. These (at least) two vaporized streams are then compressed in a CO CO2 compressor the streams at the lowest pressures being injected into the compressor further upstream than the other streams of CO2.
The principles of partial condensation and of distillation are based on the partial pressure of each constituent of the gas. In the context of CO2 capture and purification, the partial pressure of CO2 at the inlet to the cryogenic part of the method is thus a key parameter in the sizing of the separation equipment.
For example, the lower the partial pressure of CO2:                the more the gas will need to be cooled in order to begin to condense it, which means to say in order to begin to form the CO2-enriched liquid phase        as a result, the distribution of the liquid CO2 between the low-pressure streams and those not being expanded will need to be adjusted: the flow rates of expanded liquid CO2 will be increased in order to supply more low-temperature frigories        for the same ultimate cooled temperature, the lower the CO2 recovery efficiency will be        
and vice versa.
If the composition of the gas that is to be purified changes, the partial pressure of CO2 will change. That modifies the exchanges of heat needed and thus the distribution of the “low” and “medium” pressure streams of CO2 that are needed to supply the correct amounts of cold at the various temperatures. Hence, the sizing of the CO2 compressor will therefore need to take into account the fact that the distribution between the lower-pressure fluids and the other fluids resulting from the vaporization of the liquid CO2 has changed. That will lead to oversizing and mean that the operation of the compressor is not optimal. For example, if the CO2 composition of the gas that is to be purified drops with respect to the nominal CO2 composition, the compressor will need to be capable of compressing the low-pressure flow rate that is increased over and above its nominal value. The compressor will therefore need to be sized for the operating scenario of the low composition, by increasing its capacity. In nominal operation, when the composition is nominal, the flow rate sent to the compressor will therefore be well within its capacities, so its operation will therefore not be optimal.
In addition, it may happen that the liquid streams of CO2 are vaporized in exchangers which are distinct from one another according to their pressure and/or temperature following expansion. As a result, the change in distribution between the various fluids which will need to be vaporized will have an impact on the performance of these heat exchangers. According to the same example as before, the exchanger in which the lowest-pressure fluid vaporizes will need to be sized for the low-composition operating scenario and will therefore be oversized for the scenario of operating with the nominal composition. The reverse is true of the other exchanger regarding the other level or levels of vaporization: it will be oversized for operation at low composition.